The structure of a conventional power lead (also referred to as a current lead) used for providing an excitation current to a superconducting coil for magnetization is described below, with reference to FIG. 7 which illustrates schematically the structure of a conventional power lead cooled by a refrigerant gas.
Referring to FIG. 7, a superconducting coil is accommodated in a low temperature region (space) (a very low temperature chamber) such as a liquid helium (4.2.degree. K) while a power supply is placed in a room temperature environment. A power lead is provided between the superconducting coil and the power supply wherein one end of each power lead is connected to an associated end of the superconducting coil and the other end is connected to an associated electrode of the power supply.
The power lead is made up of a normal conductor such as oxygen free copper (OFCu), having a high electric conductivity. However, since the normal conductor has also high thermal conductivity, heat flows easily into the low temperature from the room temperature environment through the power lead.
Therefore, as shown in FIG. 7, cooling of the power lead by a refrigerant gas such as a helium gas is needed in a superconducting coil device.
A refrigerant gas (helium gas) flowing through the transition region from the low temperature region to the room temperature environment performing such functionalities as (1) reducing an electric resistance of the power lead as a result of which causing a reduction in a joule heat, and (2) exchanging heat flowing in from the room temperature environment to exhaust the heat to an outside.
In order to enhance a heat exchange effectiveness the power lead is usually made to have a structure having a largest possible surface area, such as a mesh or a spiral tubing. FIG. 7 illustrates the structure of a power lead with a mesh (shown as dotted lines).
A heater heats up the liquid helium and forcefully produce helium gas as a refrigerant gas. The liquid helium is supplied through a helium inlet.
Referring to FIG. 16, a conventional power lead cooled by a refrigerant gas is described in detail below. FIG. 16 illustrates a sectional view of a superconducting device comprising a superconducting coil. Since a liquid helium, which is essential to operating the superconducting coil, is expensive, it is preferable to allow the liquid helium to evaporate as little as possible and to prevent heat from coming into a superconducting coil through the current lead (also referred to as a power lead).
Referring to FIG. 16, a superconducting coil 2 is housed inside a cryostat 1, a peripheral wall of which is surrounded by a liquid nitrogen shield 13 made up of double cylinders in order to block any heat (radiation heat) coming in from a radially outside periphery. The liquid nitrogen shield 13, in turn, is accommodated within a vacuum container 15 to form a vacuum heat insulation layer. The liquid nitrogen shield 13 contains a liquid nitrogen 31 and a low temperature nitrogen gas 32 evaporated from the liquid nitrogen 31. The nitrogen gas 32 is emitted into an atmosphere through a nitrogen gas outlet 14 provided on a top of the liquid nitrogen shield 13.
The superconducting coil 2 is connected via a leader line 20 to one end of a lead conductor 3a, a component of current lead 3, while other end of a lead conductor 3a is connected to a terminal 3b located in the room temperature environment. In order to eliminate a joule heat generated in the lead conductor 3a and to prevent a heat from penetrating from a room temperature region into a very low temperature region caused by heat conduction through the lead conductor 3a, a cold helium gas 23 produced by an evaporation of a liquid helium 22 is leaded into a lead tube 3c surrounding the lead 3a to flow the helium gas 23 through a gap 3d between the lead conductor 3a and an inside of the lead tube 3c to cool the lead conductor 3a. After cooling the lead conductor 3a, the helium gas 23, branching from a top of the lead tube 3c, flows into gas tubes 5, and 6, which are connected via an electrically insulating tube joint 4 to be emitted from an external tube 7 which is connected to the gas tube 6. The vacuum container 15 and cryostat 1 are electrically insulated from the lead tube 3c by means of electrical insulator 8.
Referring to FIG. 8, a power lead made up of a high temperature superconducting material is described in the below.
The power lead, shown in FIG. 8, comprises a high temperature superconducting member made up of such as a Bi series 2223 sintered material or YCBO etc. The low temperature region is cooled with a liquid helium (4.2.degree. K), while a transition region (between the low temperature and room temperature regions) in which the high temperature superconducting member is accommodated, is cooled with both the liquid helium and a liquid nitrogen.
The transition region is separated from the room temperature region by a thermal anchor to be held at a temperature at which the high temperature superconducting member is set to a superconducting status. The thermal anchor is made of a material having a high thermal capacity such as Cu.
The high temperature superconducting member of the power lead shown in FIG. 8, when being cooled below 100.degree. K, does not generate a joule heat even when an electric current flows through the power lead to reduce an amount of heat flux penetrating into the low temperature region.
This structure shown in FIG. 8 has a meritorious effect that the entire high temperature superconducting member may be kept cooled below its critical temperature Tc by an electric conduction instead of by employing a refrigerant gas. A current lead (power lead) comprising an oxide high temperature superconducting material, which does not need a gas cooling system, has been proposed to reduce an amount of a heat flux penetrating into a low temperature region. (For example, see Yokoyama, et al; "Conduction Cooling for a Crystron; Development of a Superconducting Magnet; Design and Test of Oxide Superconductor Current Leads", Proceedings for the Fifty-Second Joint Meeting of Low Temperature Engineering and Superconductor Physics, Autumn 1993, page 235.)